In conventional air-conditioning apparatuses, R22, an HCFC refrigerant or R410A, an HFC refrigerant has been used. However, from viewpoints of protection of the ozone layer and suppression of global warming, attention has recently been paid to substitution with R32, an HFC refrigerant and R290 (propane), an HO refrigerant. The R32 serving as the HFC refrigerant and R290 (propane) of the HO refrigerant have features of considerably lower global warming potentials thereof (hereinafter referred to as GWP) relating to atmospheric release are considerably lower than those of R22, R410A, and the like.
Since the R32 refrigerant, the R290 refrigerant, and so on are flammable, there is a need to design products with sufficient consideration for safety. Since the R32 refrigerant is less flammable than the R290 refrigerant, it allows products to be designed comparatively similarly to nonflammable refrigerants R22 and R410A. However, since the R32 refrigerant is flammable, it is necessary to design products in consideration of safety. For this reason, when a flammable refrigerant is used, the cost is increased to improve safety.
In a typical air-conditioning apparatus, a compression refrigeration cycle is configured by connecting a compressor, an outdoor heat exchanger, an indoor heat exchanger, a pressure reducing device, and so on by a refrigerant pipe. Attempts have been made to use a refrigerant having a low global waving potential to effectively utilize high energy consumption efficiency, which is a characteristic of the refrigeration cycle, and to suppress global warming not only during use but also in disposal of products.
When heating operation is performed in an environment where the outside air temperature is low, such as a cold region and a snowfall region, since the heating operation is continued with increased heating capacity, the outdoor heat exchanger is frosted, and this significantly reduces heat exchange performance. For this reason, defrosting operation is periodically performed to melt frost deposited on the outdoor unit. However, when the outside air temperature is lower than or equal to the freezing point, during defrosting, drain water freezes before being drained outside through a drain outlet provided in a bottom plate of the outdoor unit, and is sometimes not drained normally. Further, if the heating operation with increased heating capacity is continued, the amount of drain water tends to increase during defrosting.
If such a state is repeated for a long time, the area where the drain water freezes extends, and the frozen drain water covers the lower part of the outdoor heat exchanger. If the area is further extended, the refrigerant pipe is broken by the volume expansion effect caused when the drain water in the lower part of the outdoor heat exchanger freezes, and this may cause refrigerant leakage. For this reason, in the regions where the outside air temperature is low, such as the cold region and the snowfall region, the air-conditioning apparatuses, around which the outside air temperature is low, is not so widespread, but there has been a tendency to use burning heating apparatuses that accelerate global warming.
There has been proposed an outdoor unit for an air-conditioning apparatus, in which a bottom plate of the outdoor unit is provided with an electric heater to suppress freezing of the bottom plate (see, for example, Patent Literature 1).